Unpin (= intentionally releasing a pinned piece for other benefits) certainly can form an essential part of a theme by itself. However there are four specially named themes for unpin operation.
Before going further it may be of interest to think how pin and unpin operations may arise in a problem.

A pin may arise:

By white action:
* Directly by white moving a proper line piece into a pinning line.
* Indirectly by white withdrawing the mask piece of a battery on a pinning line.

By black action:
* Directly by black interposing against check or capturing a white unit on a pinning line (self-pin).
* Indirectly by black withdrawing one of the half-pinned pieces.(self-pin)

An unpin may happen:(Assuming white pinning black)

* Directly by white withdrawing the pinning linepiece.
* Indirectly by white interposing a different unit between black king and the pinning linepiece. (On either side of the pinned black unit.)

There are following named themes utilizing the unpin operation:

(1) Gamage Unpin, where a black piece is already pinned in the initial position or shortly will be pinned. White then makes a move that threatens mate by direct unpin, but black has defences that defeat this threat. However, (at least some) of these defences have harmful side effects, that allow a different direct unpin resulting in a mate.

(2) Goethart Unpin is similar to the above, but now white's unpin is indirect (interposing on pinning line) in nature.

(3) Nievelt theme sees black, in response to a white threat to self-pin himself. Black dares to do this, because white's threat would directly unpin his self-pinned piece for defence. However, white is able to utilize the self-pin with other ways.

(4) Schiffman theme is similar black self-pinning theme, but here the white threat would indirectly unpin the black self-pinned piece. Again white uses some other methods to mate.

These rather abstract descriptions hopefully come clearer with tutorial examples of #1684-#1693_________________There are only three kinds of chessplayers - those who can count and those who cannot....